1. Field of the Invention
The present invention relates to an ink jet recorder.
2. Related Background Art
Many systems for the ink jet recording have been known. They are classified into three major classes, that is, (1) continuous jet type, (2) impulse type (on-demand type) and (3) electrostatic attraction type.
In the continuous jet type, continuously discharged ink is charged and deflected to record data. Accordingly, the recorder is complex and requires recovery of ink and a cleaning device. Such type of recorder is disclosed in U.S. Pat. Nos. 3,298,030 or 3,596,275.
In the electrostatic attraction type recorder, the structure is relatively simple but requires a high voltage. Accordingly, there is a problem in energy saving and safety. Further, the number of materials which can be used as ink is restricted in view of the necessity that it exhibit conductivity, and frequency response is poor. Such type of recorder is disclosed in U.S. Pat. No. 3060429.
On the other hand, in the on-demand type recorder, an ink droplet is discharged by a discharge energy supplied by energy generation means such as an electro-mechanical transducer or electro-thermal transducer only when it is required. Accordingly, the structure is very simple and suitable for the recorder. Such type recorder is disclosed in U.S Pat. Nos. 3,683,212, 3,832,579, No. 3,747,120, and No. 3,946,398.
However, as shown in FIG. 1, in the on-demand type ink jet recorder, particularly that which uses a piezoelectric electro-mechanical element as the energy generation means, a resonance frequency exists in a discharge velocity of the ink droplet in a high drive frequency range. If the ink droplet is discharged at such a resonance frequency, the discharge state is very unstable.
A reason for such a resonance frequency may be that a pressure wave generated by the piezoelectric element, when the ink droplet is discharged acts not only toward the nozzle 1 (in the direction of discharge of the ink droplet) but also in the opposite direction, toward the ink supply path. This pressure wave is reflected at the rear and the reflected wave thus affects to the discharge state of the next ink droplet.
Accordingly, by observing a meniscus after the ink discharge, the presence of the pressure wave is recognized. FIG. 2 illustrates meniscus vibration. The local unevenness of a characteristic curve of FIG. 2 may be due to the reflection wave.
A period t of resonance is a function of the velocity of sound c in the ink in the nozzle and a length l of the nozzle, ##EQU1## It substantially corresponds to a resonance frequency measured in FIG. 1 and a period of unevenness of the curve shown in FIG. 2.
If the reflection wave is large at the point R in FIG. 2, the vibrating meniscus moves past the orifice and a required ink droplet 101 as well as an extraneous droplet 102 are discharged from the head end 103 as shown in FIG. 3. Such a discharge state is very unstable and the droplet 102 degrades the print quality. Accordingly, those problems must be solved.
In order to stabilize the discharge of the ink droplet, it is necessary to prevent the reflection wave from moving toward the front of the nozzle. To this end, the pressure wave propagated toward the back of the nozzle and the reflection wave should be attenuated in the ink. Such attenuation may be attained by increasing the viscosity of the ink or increasing the length of the nozzle. In both methods, the pressure wave is attenuated but the viscosity resistance in the nozzle increases or the frequency response is degraded.
In the past, the frequency response is weighted and the ink viscosity is selected rather low and the nozzle length is selected rather short. As a result, the affect of the reflection wave is significant and the stability of the discharge of the ink droplet is not good.